The fundamental bounds proposed in this paper are based on an idea originally used to bound the performance of
antennas [1]}, [2]}, [3]} and recently broadened to cover extremal
optical cross-sections [4]}, [5]} or radiative heat transfer [6]}, [7]}, see [8]} for a recent review of the topic and an extensive list of references. The basis of this framework is the use of field quantities as optimized variables and is typically connected to electromagnetic field descriptions via field integral equations [9]}, [10]}, [11]}, and quadratically constrained quadratic program [12]}, [13]}.
